In mechanical engineering, tolerances set the allowable deviation from assigned dimensions. The use of tolerances helps to ensure that the final product is readily usable, especially if it is a part of a larger assembly.
Not setting a tolerance in a critical area may render the part unusable according to the design intent, as each fabrication method comes with a certain level of inaccuracy.
However, pinpointing a suitable tolerance makes sure that the manufacturing company knows to tackle a few specific points in the production process with more attention. This can be the difference between perfectly mating parts and scrap metal.
What is Tolerance in Engineering?
Engineering tolerance is the permissible variation in measurements deriving from the base measurement.
Tolerances can apply to many different units. For example, the working conditions may have tolerances for temperature (° C), humidity (g/m3), etc. In mechanical engineering, we are mainly talking about tolerances that apply to linear, angular, and other physical dimensions.
But regardless of the unit, a tolerance states an acceptable measurement range from the base point (nominal value). Let’s say you are designing a sieve to separate 3.5 mm pebbles from 2.5 mm pebbles. You want the smaller pebbles to fall through the holes while keeping the larger ones on the sift.
The larger pieces of rocks vary in size between 3.3 mm and 3.7 mm. The smaller ones are in the range of 2.3…2.7 mm. To ensure that only the smaller ones, all of them, will actually fall through the holes while keeping the larger ones on the sift, you can set the nominal value for the hole diameter as 2.8 mm. At the same time, manufacturing accuracy will mean that you may end up with some holes at 2.6 mm.
Adding a lower limit of -0 mm and an upper limit of +0.3 mm guarantees that all the holes will be between 2.8 and 3.1 mm in diameter.
Engineering tolerance is the permissible limit or limits of variation in:
- a physical dimension;
- a measured value or physical property of a material, manufactured object, system, or service;
- other measured values (such as temperature, humidity, etc.);
- in engineering and safety, a physical distance or space (tolerance), as in a truck (lorry), train or boat under a bridge as well as a train in a tunnel (see structure gauge and loading gauge);
- in mechanical engineering the space between a bolt and a nut or a hole, etc.
Dimensions, properties, or conditions may have some variation without significantly affecting the functioning of systems, machines, structures, etc. A variation beyond the tolerance (for example, a temperature that is too hot or too cold) is said to be noncompliant, rejected, or exceeding the tolerance.
Considerations when setting tolerances
A primary concern is to determine how wide the tolerances may be without affecting other factors or the outcome of a process. This can be done by the use of scientific principles, engineering knowledge, and professional experience. An experimental investigation is very useful to investigate the effects of tolerances: Design of experiments, formal engineering evaluations, etc.
A good set of engineering tolerances in a specification, by itself, does not imply that compliance with those tolerances will be achieved. The actual production of any product (or operation of any system) involves some inherent variation of input and output.
Measurement error and statistical uncertainty are also present in all measurements. With a normal distribution, the tails of measured values may extend well beyond plus and minus three standard deviations from the process average. Appreciable portions of one (or both) tails might extend beyond the specified tolerance.
The processing capability of systems, materials, and products needs to be compatible with the specified engineering tolerances. Process controls must be in place and an effective Quality management system, such as Total Quality Management, needs to keep actual production within the desired tolerances.
A process capability index is used to indicate the relationship between tolerances and actual measured production.
The choice of tolerances is also affected by the intended statistical sampling plan and its characteristics such as the Acceptable Quality Level. This relates to the question of whether tolerances must be extremely rigid (high confidence in 100% conformance) or whether some small percentage of being out-of-tolerance may sometimes be acceptable.
Types of Tolerances
Today, there are 14 types of geometric tolerances by the number of symbols, and 15 types based on classification.
These are grouped into Form tolerance, Orientation tolerance, Location tolerance, and Run-out tolerance, which can be used to indicate all shapes.
Following are the three types of tolerances used in measurements:
- Unilateral tolerances
- Bilateral tolerances
- Compound tolerances.
1. Unilateral Tolerances
When the two limit dimensions are only above the nominal size as shown in the figure or only below the nominal size then the tolerance is said to be unilateral.
2. Bilateral Tolerances
When the two-limit dimension is above and below the nominal size, Then the tolerances are said to be bilateral.
3. Compound Tolerances
Compound tolerance is determined by the established tolerances i.e., the combination of more than one type of tolerances are called compound tolerances, the different types of tolerances may be angular, lateral, etc.
For Example: In figure tolerances on dimension/are dependent on tolerances of L, h, and θ. This compound tolerance on ‘l’ is the combined effect of these three tolerances. The minimum tolerance on ‘l’ will be corresponding to L-b, θ+∝ , and h+c.